Astronomy & Astrophysics manuscript no. iiizw2
February 2, 2008
(DOI: will be inserted by hand later)
The extreme flare in III Zw 2:
Evolution of a radio jet in a Seyfert galaxy
A. Brunthaler 1 ' 2 , H. Falcke 3 ' 4 , G.C. Bower 5 , M.F. Aller 6 , H.D. Aller 6 , and H. Terasranta 7
1 Max-Planck-Institut fur Radioastonomie, Auf dem Hiigel 69, 53121 Bonn, Germany
2 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
3 ASTRON, Postbus 2, 7990 AA Dwingeloo, The Netherlands
4 Department of Astrophysics, Radboud Universiteit Nijmegen, Postbus 9010, 6500 GL Nijmegen, The
Netherlands
5 Radio Astronomy Laboratory, University of California at Berkeley, 601 Campbell Hall, CA 94720, USA
6 University of Michigan, Astronomy Department, Ann Arbor, MI 48109-1090, USA
7 Metsahovi Radio Observatory, Helsinki University of Technology, Metsahovintie 114, 02540 Kylmala, Finland
Received ; accepted
Abstract. A very detailed monitoring of a radio flare in the Seyfert I galaxy III Zw 2 with the VLA and the
VLBA is presented. The relative astrometry in the VLBA observations was precise on a level of a few ^as. Spectral
and spatial evolution of the source are closely linked and these observations allowed us to study in great detail a
textbook example of a synchrotron self-absorbed jet. We observe a phase where the jet gets frustrated, without
expansion and no spectral evolution. Then the jet breaks free and starts to expand with apparent superluminal
motion. This expansion is accompanied by a strong spectral evolution. The results are a good confirmation of
synchrotron theory and equipartition for jets.
Key words, galaxies: active - galaxies: individual (III Zw 2) - galaxies: jets - galaxies: Seyfert -
However, a few sources with intermediate radio-
to-optical ratios appear to be neither radio-loud nor
radio-quiet. They form a distinct subclass with very
similar radio morphological and spectral properties. They
all have a compact core at arcsecond scales and a flat
and variable spectrum in common. These properties are
very similar to the ones of radio cores in radio-loud
quasars, but their low radio-to-optical ratio and their
low extended steep-spectrum emission is atypical for
radio-loud quasars. Miller, Rawlings, & Saunders (19931
and Falcke, Sherwood, & Patnaik (19961 have iden-
tified a number of these sources, called "radio-
intermediate quasars" (RIQs), and suggested that
they might be relativistically boosted radio-weak
quasars or "radio- weak blazars". This would im-
ply that most, if not all, radio-quiet quasars also
have relativistic jets. In fact, Very Long Baseline
Interferometry (VLBI) observations of radio-quiet
quasars already have shown high-brightness temperature
radio cores and jets (Falcke, Patnaik, & Sherwood 1996
1. Introduction
The radio properties of quasars with otherwise very sim-
ilar optical properties can be markedly different. There
is a clear dichotomy between radio-loud and radio-quiet
quasars in optically selected samples. The radio-loudness
is usually characterized by the radio-to-optical flux ra-
tio. In the PG quasar sample, which is probably the
best studied quasar sample in the radio and optical
IjKellermann et al. 19891 IBoroson fc Green 1992|l . radio-
loud and radio-weak quasars separate cleanly in two dis-
tinct populations (e.g. IKellermann et al. 1989).
It is known that radio-loud AGN almost never reside
in late type, i.e. spiral galaxies (e.g. IKirhakos et al. 1 999
Bahcall, Kirhakos, & Sch neider 1995| ) whereas radio-
quiet quasars appear both in spiral and in elliptical host
galaxies. Furthermore, all relativistically boosted jets
with superluminal motion and typical blazars have been
detected in early type galaxies (e.g. |Scarpa et al. 20 00).
It is still unclear, why AGN in spiral galaxies, at the same
optical luminosity as their elliptical counterparts, should
not be able to produce the powerful, relativistic jets seen
in radio galaxies.
|Blundell fc Beasley 1998| ). A crucial test of the relativistic
jet hypothesis is the search for apparent superluminal
motion in these sources. A prime candidate for detecting
Send offprint requests to: brunthaler@jive.nl
2
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
this is the brightest radio source in the RIQ sample,
III Zw 2, which we discuss in this paper.
Ill Zw 2 ( PG 0007+106 Mrk 1501, z = 0.089) was
discovered by [Zwicky (1967|), classified as a Seyfert I
galaxy (e.g., |Arp 1968| IKhachikian fc Weedman 19741
IQsterbrock 1977|L and later also included in
the PG quasar sample ( Schm idt fc Green"T9 83).
The host galaxy was classified as a spiral (e.g.
|Hutchings fc Campbell 1983| ) and a disk model was
later confirmed by fitting of model isophotes to near-IR
images dTaylor et al. 1996) . A spiral arm was claimed
(Hut chings 1983D but recent observations suggest a
tidal arm with several knots of star forming regions
dSurace, Sanders, fc Evans 200T| |. Ill Zw 2 is the bright-
est member of a group of galaxies and an extended low
surface brightness emission surrounding all the galaxies
suggests that there are perhaps interactions between the
galaxies dSurace, Sanders, fc Evans 2001| ).
The source has shown extreme variability at ra-
dio wavelengths with at least 20-fold increases in
radio flux density within 4 years IjAller et al. 1985|l .
Ill Zw 2 is also known to be variable in the
optical ([Lloyd 1984] IClements et al. 19~95| a nd X-ray
Table 1. Total observing time i b s , fraction of observing
time on III Zw 2 at 43 GHz and 15 GHz /15 and frac-
tion of observing time on phase-reference quasar f qua sar
for the VLBA observations.
IfKaastra fc de Korte 1988|l . |Salvi et al. (2002|) compare
the long term radio light curves of III Zw 2 with optical,
IR and X-ray light curves and find indications for cor-
related flux variations from radio to X-ray wavelengths.
Unfortunately, the time sampling at other wavelengths
than radio is very poor.
Ill Zw 2 is a core-dominated flat-spectrum AGN with
only a faint extended structure (see [linger e t al. 1987
and Section 13. If) . The weak extended radio emission
and the host galaxy is quite typical for a Seyfert
galaxy. Its [OIII] luminosity is a mere factor three
brighter than that of a bright Seyfert galaxy like Mrk 3
(e.g. |Alonso-Herrero, Ward, fc Kotilain en 1997| ) which ex-
plains why it has been classified as either a Seyfert galaxy
or a quasar. In this luminosity region a distinction between
the two may not be of much significance.
Earlier VLBI observations of the source have
only shown a high-brightness temperature un-
resolved core QFalcke, Sherwood, fc Patnaik 1996|
IKellermann et al. 1998~| and Millimeter- VLBI obser-
vations by Falcke et al. (1999l just barely resolved the
source into two very compact components. A broad-
band radio spectrum showed a highly peaked spectrum
which was well explained by a very compact source and
synchrotron self-absorption.
The unique and simple structure and timescales of ra-
dio outbursts within 5 years makes III Zw 2 and ideal
source to study radio-jet evolution relevant also to radio
galaxies.
In section 2, we will describe our Very Large Array
(VLA) and Very Long Baseline Array (VLBA) observa-
tions, before results from VLA monitoring are presented
in sections 3.1 - 3.3. We then describe the results from
VLBI observations in section 3.4. In section 4 we will dis-
cuss the results.
Date
^obs
/15
fquasar
1998/02/16
8 h
0.75
0.25
1998/06/13
9 h
0.75
0.25
1998/09/14
8 h
0.75
0.25
1998/12/12
8 h
0.75
0.25
1999/07/15
6 h
0.75
0.25
1999/11/12
8 h
0.33
0.33
0.33
2000/07/22
6 h
0.33
0.33
0.33
2000/08/27
8 h
0.33
0.33
0.33
2000/09/06
8 h
0.33
0.33
0.33
2. Observations
In 1996 III Zw 2 started a new major radio outburst and
we initiated a target of opportunity program to monitor
the spectral evolution of the burst with the VLA and its
structural evolution with the VLBA with excellent relative
astrometry of the component separation.
We observed III Zw 2 with the VLA 41 times from 1998
September until September 2001 in intervals of roughly
one month. The observations were made at six frequencies
ranging from 1.4 GHz to 43 GHz. Results from an obser-
vation on 1998 May 21 at 350 MHz and on 1999 July 7 at
327.5 MHz are also presented here. The source 3C48 was
used as the primary flux density calibrator, and III Zw 2
was self-calibrated and mapped with the Astronomical
Image Processing System (AIPS).
We also used the monitoring data at 8 and 15 GHz
obtained with the Michigan 26 m telescope, and at 22 and
37 GHz from the Metsahovi radio telescope. The single
dish data are important for placing the VLA and VLBA
data in context, as they include a larger time window and
are more closely spaced.
We observed III Zw 2 with the VLBA nine times
over a period of 2.5 years at 15 and 43 GHz. Details of
this observations are given in Table In the last four
epochs we included the background quasar J0011+0823 at
15 GHz as phase-reference source. For the second epoch,
we used the Effelsberg 100 m telescope in combination
with the VLBA. We observed four 8 MHz bands, each
at right and left circular polarization. The initial cali-
bration was performed with the AIPS package. A-priori
amplitude calibration was applied using system tempera-
ture measurements and standard gain curves. Fringes were
found in the III Zw 2 data on all baselines. The data
were self-calibrated and mapped using the software pack-
age DIFMAP dShepherd, Pearson, fc Taylor 1994| ). We
started with phase-only self-calibration and later included
phase-amplitude self-calibration with solution intervals
slowly decreasing down to one minute. Results of the
first five VLBA observations at 43 GHz were reported
by Brunthaler et al. (2000)
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
3
15
10
O 5
uj
CO
g o
<
-5
-10
-15
-20 -
-25
;.0
Fig. 1. Combined VLA map of 11 epochs (A,B,C and D-
array) of III Zw 2 at 1.4 GHz (top) and 4.8 GHz (bottom).
All maps were convolved with a beam of 2 x 2 arcseconds
to detect faint extended emission. The contours start at
0.26 mJy and 0.15 mJy at 1.4 GHz and 4.8 GHz respec-
tively and increase with a factor of \/2.
20
15
10
O 5
LU
CO
Z o
<
-5
-10
-15
-20
-25
15
10
O 5
LU
CO
<
-5
-10
-15
-20
-25
10
ARC SEC
Fig. 2. Combined VLA map of 11 epochs (A,B,C and D-
array) of III Zw 2 at 8.4 GHz (top) and 15 GHz (bottom).
All maps were convolved with a beam of 2 x 2 arcseconds
to detect faint extended emission. The contours start at
0.12 mJy and 0.6 mJy at 8.4 GHz and 15 GHz respectively
and increase with a factor of
3. Results
3.1. Extended emission of III Zw 2
Unger et al. (19871 discovered a weak radio component
15.4" (23 kpc, with an angular size distance of d,A ~ 307.4
Mpc; H = 75 km sec -1 Mpc~ x , qo = 0.5 as used in this
paper) southwest of the nucleus. This detection was con-
firmed later <|Kukula et al. 19981 IFalcke et al. 1999|l . but
no additional extended radio emission was found.
To study the extended structure in more detail, we
combined the raw data of eleven VLA observations. In
the combined data we used data from the VLA in A, B, C
and D configuration. Since the nucleus is highly variable,
we subtracted it from the uv-data before combining the
data. The combined data set was then self-calibrated and
mapped. The combined VLA maps at 1.4, 4.8, 8.4 and
15 GHz are shown in Fig. [2 and [3 The 4.8, 8.4 and 15
GHz maps were convolved with a large beam of 2 x 2 arc-
seconds to detect faint extended structure. We detected
the southwestern component at all four frequencies. This
radio lobe or hotspot is connected to the nucleus with a
jet-like structure visible at 1.4, 4.8 and 8.4 GHz. At 1.4
GHz one sees an indication that the jet is ejected in north-
western direction and gets deflected by almost 90° towards
the southwestern lobe. This is also in accordance with the
direction of the jet on sub-parsec scales (see section l3~4"|) .
We also discovered a weaker secondary radio lobe
21.9" (32.6 kpc) on the opposite side of the galaxy at
1.4 to 8.4 GHz. If one assumes equal expansion ve-
locities for both lobes, a simple time travel argument
(e.g., |Ryle fc Longair 1 967) would suggest that the weaker
northeastern lobe is approaching and the brighter south-
western lobe is receding. However, this scenario can not ex-
plain the differences in flux density between the two lobes.
One would expect the approaching lobe to be brighter due
to relativistic boosting of the emission.
Hence it is more likely that the armlength difference
is explained by an asymmetric expansion of the the two
lobes due to different intrinsic velocities or differences in
the ambient medium, i.e. the medium in the southwest
of III Zw 2 has a higher density than the medium in the
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
100 r
m
c
cd
"D
2.5
1 10
Frequency [GHz]
100
Fig. 3. Spectra of the southwestern (circles) and north-
eastern (squares) radio lobes. The black triangles are up-
per limits for the northeastern lobe.
northeast. This is supported by the fact that there is a
close companion galaxy only ~ 30" to the south.
The spectra of the two radio lobes are shown in Fig [21
The 350 and 327.5 MHz data were from our observa-
tions on 1998 May 21 and 1999 July 7 respectively where
we also detected the southwestern component. Both ra-
dio lobes have a steep spectrum with spectral indices of
a = —0.57 to —1.15. Values around -0.7 are typical for
synchrotron emission of optically thin radio lobes of radio
galaxies. There is no break or a steepening of the spectrum
towards higher frequencies. This indicates that both radio
lobes are still active and powered by the central engine.
Otherwise, the high energy electrons would have lost most
of their energy due to radiation losses. This would lead to
a steepening in the spectrum at higher frequencies.
3.2. Variability
The core of III Zw 2 shows extreme variability at radio
wavelengths. Long time radio light curves of this source
spanning more than 20 years are shown in Fig. 0] using
data from Michigan, Metsahovi, and the VLA. One can
see major flares with 30-fold increases in radio flux density
within two years. These major flares occur roughly every
five years with sub-flares on shorter timescales.
The outburst discussed in this paper started in 1996
and we monitored this flare with the VLA. The good time
sampling of one observation each month allowed us to
study this outburst in great detail. Lightcurves from the
most recent flare at six frequencies from 1.4 to 43 GHz are
shown in Fig.ElandEltogether with our best model fits to
the data. Since the Michigan data at 8 GHz is rather noisy
compared to the other frequencies and the VLA data, we
used only our VLA monitoring data for the fits at this
frequency. At 15, 22 and 37 GHz we used the VLA data
as well as the Michigan and Metsahovi data.
First we fitted a linear rise and decay to the flare. The
rise is consistent with a linear fit at all frequencies. The
CD
"O
CD
"D
CD
"D
1.5 -
0.5
3.5
2.5
& 2 h
CD
■o 1.5
0.5
3.5
I I
8 GHz
1
1
1 1
- eA A
- f i •
» ^ A A
A &
* *
i * A 4
1 1
A |
1
a 1 N»
« 1 !
2.5
2
1.5
1 h
0.5
3.5
3
2.5 h
2
1.5
1
0.5
~i r
, 15 GHz
"T"
22 GHz
37/43 GHz
5.
•g f
1980
1985 1990
Date [years]
1995
2000
Fig. 4. Radio light curves of III Zw 2. The triangles at 8
and 15 GHz are from the Michigan monitoring program,
the squares at 22 and 37 GHz are from the Metsahovi
monitoring program. The circles are our VLA observations
cd
"O
CO
CD
T5
CO
CD
"O
0.2
0.1
0.5
0.4
0.3
0.2 -
0.1
0.9
0.8
0.7
0.6
0.5 h
0.4
0.3
0.2 h
0.1
1997
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
1.8
1.4 GHz
4.8 GHz
~i r
8.4 GHz -
1998
1999 2000
Date [years]
2001 2002
Fig. 5. Radio light curves of the recent flare in III Zw 2
at 1.4, 4.8 and 8.4 GHz. The circles are our VLA observa-
tions, and the triangles are from the Michigan monitoring
program. The solid lines are our linear rise and decline
fits. The dashed lines are the fitted exponential rise and
decay.
CD
"a
CO
cz
CD
"a
CO
cz
CD
■a
x
1.5
0.5
I I
1
1 1
22 GHz
Isp □ ffl
□ Jp s
a APT
\
\ \
\\
\ •
\ \*
\
□
- /
□ei/
1 1
1
1 1
1997
1998
1999 2000
Date [years]
2001
2002
Fig. 6. Radio light curves of the recent flare in III Zw 2 at
15, 22 and 37/43 GHz. The circles are our VLA observa-
tions, and the triangles are from the Michigan monitoring
program and the squares are from the Metsahovi monitor-
ing program. The solid lines are our linear rise and decline
fits. The dashed lines are the fitted exponential rise and
decay.
decay is also linear at 4.8 GHz. At higher frequencies, the
decay is linear only for a short time period and deviates
significant from a linear behavior at later times. Thus we
used only the linear part of the lightcurves for our fits.
The decay at higher frequencies can be fitted much
better with an exponential decay. Thus we fitted also an
exponential rise and decay,
S(t) = S e ( *°~ t)/Tp , (1)
S(t) = S e {t "- t)/Td , (2)
to the lightcurves. The fitting parameters are So, the max-
imum amplitude of the flare and the flare rise and decay
timcscales r r and r<j. The epochs of the flare maximum
to were taken from the observation with the highest flux
density. The increase of the lightcurves can also be fitted
by an exponential rise and the exponential decay fits the
outburst until a new smaller flare starts.
6
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
Table 2. Fitting information: The slopes of the linear
rise (a r ) and decay (ad), the exponential rise and decay
timescales t t and Td and the epoch of the flare maximum
t .
Frequency
a r
Td
to
[GHz]
[Jy yr
- 1 ]
[yr
[yr]
1.4
0.021
-3.088
4.8
0.20
-0.14
-1.39
2.16
2000.1
8.4
0.46
-0.43
-1.40
1.26
1999.5
15
0.74
-0.86
-1.14
1.01
1999.1
22
1.24
-2.85
-1.04
0.73
1998.9
43
1.65
-4.73
-0.96
0.53
1998.7
m
CD
E
10
Frequency [GHz]
100
Fig. 7. Time-lag between the peak at 43 GHz and the
peak at the other frequencies. The data point at 1.4 GHz
is a lower limit.
The slopes of the linear rise (a r ) and decay (ad), the
exponential rise and decay timescales r r and Td and the
epoch of the flare maximum to are listed in Table [2 for all
six frequencies.
The lightcurve reaches its peak first at higher fre-
quencies. The time-lag between the peak at 43 GHz
and the peak at lower frequencies is shown in Fig.
and is best fit with a St oc v~ 1A power law. The rise
and decay are faster at higher frequencies than at lower
frequencies. This behavior is typical for flares in AGN
(e.g. |Turler, Courvoisier, fc Paltani 1999| ).
At 4.8 GHz the rise is faster than the decay, while at
15, 22 and 43 GHz the rise is slower than the decay. This
is the case for the linear and the exponential fits to the
flare. At 8.4 GHz the linear rise is slightly slower than the
linear decay while the exponential rise slightly faster than
the exponential decay is. The rise and decay timescales are
plotted as a function of frequency in Fig. |H1 The data can
be fitted with power laws r r oc v~ 2 and Td oc i/~ 7 . Only
the rise timescale at 1.4 GHz deviates from this power law.
However, the quiescence flux at 1.4 GHz is comparable to
10 100
Frequency [GHz]
Fig. 8. Rise (x) and decay (+) timescales for all frequen-
cies.
the flux density of the outburst and will affect this data
point.
The small dependancy of the rise timescale on fre-
quency indicates that optical depth effects are not very
important during the rise. This is in contrast to the de-
cay, where optical depth effects are clearly important.
Valt aoja et a l. (1999) modeled the radio lightcurves of
flares at 22 and 37 GHz in 85 extragalactic radio sources.
They fitted an exponential curve to the rise and the decay
and found that in virtually all flares a good model-fit could
be obtained using a constant ratio between decay and rise
timescale of Td = 1.3r r , i.e. the rise is faster than the
decay. These flares can be identified with the ejection of
new VLBI components in the jets.
The fact that the lightcurves of III Zw 2 at 15, 22
and 43 GHz show the opposite behavior with ratios of
Td/r r « 0.6 — 0.9, i.e. the decay is faster than the rise,
indicates that different physical processes are involved in
this source. However, it can not be excluded that the flare
is composed of two closely spaced flares.
3.3. Spectral evolution
During the outburst the spectrum of the source also
showed variations. An almost simultaneous broadband
radio spectrum from 1.4 to 660 GHz during the in-
crease in flux density in May 1998 was presented in
Falcke et al. (1999 1. The spectrum was highly inverted at
centimeter wavelengths (a — +1.9 ± 0.1) with a turnover
frequency around 43 GHz. At frequencies above 43 GHz
the spectrum became steep with a spectral index of a =
—0.75 ± 0.15, i.e. a textbook-like synchrotron spectrum.
Our VLA monitoring of the spectral evolution started
in September 1998. Four of the 41 epochs yielded no or
bad data due to bad weather or hardware failure. We fitted
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
7
10
Frequency [GHz]
100
Fig. 9. Spectra of III Zw 2 on 1998 November 04 (top),
1999 February 22 (middle), 1999 November 12 (middle)
and 2001 January 30 (bottom) together with the quiescent
spectrum (horizontal line) and the best fit spectrum.
the remaining spectra with a broken power-law plus a flat
and constant quiescence spectrum S q ,
S(v) = S -
1 - e"
S,
(3)
where k and I are the spectral indices of the rising and
declining parts of the spectrum. Sq and vq are fitting pa-
rameters and are not exactly equal to the maximum flux
density and the peak frequency of the fitted spectrum.
We assume a flat spectrum for the quiescence flux
which is typical for quiescent cores in active galactic nu-
clei. Since we could fit all epochs with a value of 25 mJy,
we adopted this value for all epochs.
The approach with a broken power-law has the dis-
advantage, that there are ambiguities in the parameters
if the peak of the spectrum falls beyond the frequencies
covered in the observations.
This situation was the case in our first three obser-
vations in September 1998, October 1998 and November
1998. The spectral shape in these three epochs was very
similar to the May 1998 spectrum. Highly inverted at cen-
timeter wavelengths, a flattening towards higher frequen-
cies, and possibly a turnover around 43 GHz, but with
higher flux densities (see Fig. top) . Thus we assumed
the spectral index of the declining part of the spectrum
to be / = —0.75, the value of the May 1998 observation.
This is a reasonable assumption since the overall spectral
shape did not change significantly between May 1998 and
September 1998.
The temporal evolution of the fitting parameters vq, k
and I can be seen in Fig. II 01 and Fig ^2 In some epochs
we covered only 5 frequencies from 1.4 to 22 GHz. The
absence of the 43 GHz flux density in these epochs could
bias the results of the spectral fitting. Thus we marked
the epochs with only 5 frequencies in Fig. 1101 with trian-
gles while the epochs with 6 frequencies are indicated by
circles. One can see that the fits of both subsets are in
good agreement.
After November 1998 the spectrum underwent a dra-
matic change (see also Fig.EJl. The turnover frequency vq,
that stayed roughly constant at around 30 GHz from May
1998 until November 1998 dropped to 23 GHz in December
1998. In the following months, the turnover drops further
until it reached 10 GHz in June 1999. During the next year
the turnover frequency showed only smaller and slower
variations and stayed roughly constant at ~ 7 GHz. The
drop in turnover frequency to 4 GHz in the first months
of 2000 can be explained by the onset of a new minor flare
at high frequencies (see 43 GHz lightcurve in Fig. . The
new flare caused a flattening of the optical thin part of the
spectrum and a shift of the turnover to lower frequencies.
The flattening can also be seen in Fig. Illlwhere the spec-
tral index I changes from ~ —0.9 to ~ —0.1 during that
time.
The spectral index in the optical thick part of the spec-
trum k stays at ~ 2 and slowly flattens towards later
times.
8
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
35
30
25
20
15
10
5
i r
o
oo
o
o
o
°o <sP o a □
A □
OO
1998 1998.5 1999 1999.5 2000 2000.5 2001 2001.5
Date [years]
Fig. 10. Evolution of the "turnover frequency" v$. The
circles mark epochs with 6 observed frequencies (1.4-43
GHz) and the triangles epochs with 5 observed frequencies
(1.4-22 GHz). In the last two epochs only 4 frequencies
(1.4-15 GHz) were used because of a new outburst at high
frequencies. The cross is the turnover frequency of the
spectrum in May 1998, where the 1-43 GHz data is taken
from Falcke et al. (1999 k The asterisks mark the epochs
of our VLB A observations.
CD
CO
oo n c9° °
1 r
o
o o o
OoO° o
o o
□
□
2.5
2
1.5
1
0.5
-0.5
-1
-1.5
1998 1998.5 1999 1999.5 2000 2000.5 2001 2001.5
Date [years]
Fig. 11. Evolution of the spectral indices k (circles) and
1 (squares). The asterisks mark the epochs of our VLB A
observations.
A new strong outburst started at high frequencies in
January 2001 (see 22 and 43 GHz lightcurves in Fig.|SJ and
one would have to model two independent broken power-
laws to the spectrum. Since one broken power-law is char-
acterized by four parameters So, vo, k and I, our six data
points in each spectrum are not sufficient to model two in-
dependent components with four parameters each. In the
first two epochs of the new flare, only the 22 and 43 GHz
data were affected and we fitted the broken power-law to
the remaining four frequencies.
The fast change in peak frequency implies also a strong
morphological change, i.e. a rapid expansion. This predic-
Table 3. Flux densities S, separation D and position angle
P.A. of the two outermost point-like components of our
model-fits to the 43 GHz uv-data.
Date
Si [Jy]
S 2 [Jy]
Ss [Jy]
d [mas]
P.A.
1998/02/16
0.93
0.58
0.075
-84°
1998/06/13
1.03
0.65
0.077
-78°
1998/09/14
1.60
1.27
0.077
-72°
1998/12/12
0.86
0.86
0.106
-63°
1999/07/15
0.56
0.26
0.08
0.245
-71°
1999/11/12
0.08
0.23
0.05
0.246
-73°
tion was tested by VLBI observations which are described
in the next section.
3.4. Structural evolution
3.4.1. 43 GHz Results
The first three VLBA observations were made during the
first phase of the flare, marked by the increase in flux
density and a roughly constant spectral peak above 30
GHz. The constancy of the peak frequency indicates no
structural change, since the turnover is caused by syn-
chrotron self-absorption (Fal cke et al. 1 999). The source
is slightly resolved and the long baselines show non-zero
closure phases, indicating an asymmetric structure. Two
point-like components were fitted to the uv-data to repre-
sent the extent of the source. The source shows no struc-
tural change (see Fig. I12|) and the separation of the two
components during this phase of the flare stayed constant
at ~ 76 [ias, corresponding to ~ 0.11 pc. The excellent
agreement within 2 /ias between the first three epochs
shows the high quality of the data and the accuracy of
the relative astromctry.
After November 1998, the VLA monitoring shows a
dramatic change in the spectrum. The peak frequency
dropped quickly to 10 GHz within a few month (Fig. I10|) .
In the framework of a simple equipartition jet model with
a R tx v~l dependence (e.g., |Blandford k. Konigl 1979|
Falcke fc Biermann 1995)) one would expect a rapid ex-
pansion. With a source size of 0.11 pc and a turnover
frequency of 33 GHz in the first phase with no expansion
one expects a source size of 0.36 pc for a self- absorption
frequency of 10 GHz according to the spectral evolution.
Indeed, the fourth VLBA epoch, observed only one
month after the start of the spectral evolution, shows
first signs of an expansion. The fifth epoch shows a dra-
matic structural change (see Fig. [T2j l and a model of three
point-like components is required to describe the data
now. The separation between the outer components is now
~ 245 [ias corresponding to ~ 0.37 pc. This is in good
agreement with the expected value of 0.36 pc from the
equipartition jet model. The structure in the sixth epoch
is very similar to the fifth epoch but with lower flux den-
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
9
Contours: 2.5 mjy*(-l, 1,2,4,8,16,32,64)
,
O 1998/02/16
Peak flux = 1.27 Jy/beam
Contours: 4 mjy*(-l, 1,2,4,8,16,32,64)
1998/06/13
Peak flux = 1 .42 Jy/beam
Contours: 5 mjy*(-l, 1,2,4,8,16,32.64)
1998/09/14
Peak flux = 2.4 Jy/beam
Contours: 2 mjy*(-l,l,2,4,8,16,32,64) r .
r\
1998/12/12
Peak flux = 1.29 Jy/beam
Contours: 2.5 mJy*(-l, 1,2,4,8,16,32,64)
o
1999/07/15
Peak flux = 0.62 Jy/beam
Contours: 2.3 mjy*(-l, 1,2,4,8,16,32,64)
1999/11/12
Peak flux = 0.23 Jy/beam
(mas)
Fig. 12. All six VLB A maps of III Zw 2 at 43 GHz con-
volved with a superresolved circular beam of 150 fi&s.
sity. This is again expected, since the turnover frequency
stayed around 10 GHz.
The separation of the outer components for all six
epochs is plotted in Fig. ^| (upper panel). For the first
three epochs we measure an upper limit for the expansion
speed of 0.04 c. The rapid expansion between the fourth
and fifth epoch shows an apparent speed of 1.25 c. Between
the last two epoch we detected again no expansion with
an upper limit of 0.04 c.
Unfortunately the source was too weak at 43 GHz to
be detected at the last three epochs.
3.4.2. 15 GHz Results
At 15 GHz the picture looks completely different. The
source is very compact but slightly resolved in all epochs
except the first and we fitted two point-like components
to the uv-data. The flux densities, separations and posi-
tion angles of the two components are listed in Table 0]
The component separation of all epochs is also plotted in
Fig. El One can see a constant expansion with an ap-
parent expansion speed of ~ 0.6 c. Simple extrapolation
backwards suggests that the expansion has started in May
1996. This is consistent with the onset of the new flare in
Table 4. Flux densities S, separation D and position angle
P.A. of the two point-like components of our model-fits to
the 15 GHz uv-data.
Date
Si [Jy]
S 2 [Jy]
d [mas]
P.A.
1998/02/16
0.32
0.40
0.085
-54°
1998/06/13
0.45
0.48
0.114
-79°
1998/09/14
0.67
0.50
0.121
-70°
1998/12/12
0.79
0.69
0.145
-59°
1999/07/15
0.50
0.46
0.142
-87°
1999/11/12
0.43
0.25
0.195
-68°
2000/07/22
0.22
0.10
0.222
-61°
2000/08/27
0.19
0.08
0.220
-68°
2000/09/06
0.18
0.07
0.229
-63°
the 37 GHz lightcurve in Fig.0J The flare started between
two 37 GHz observations in May and October 1996.
One should note that the component separation of the
fifth epoch shows a deviation from a constant expansion.
If one splits up the separation into its north-south and
east-west components the scatter is larger in the north-
south direction. This is expected, since the beam of the
VLBA is elongated in the north-south direction. This rel-
atively large scatter in north-south direction also explains
the scatter in the positions angles in Table.
4. Discussion
The stop-and-go behavior and the apparent contradiction
between the 43 GHz and 15 GHz data can be explained
by a jet interacting with the interstellar medium in com-
bination with optical depth effects in an 'inflating-balloon
model'.
In this model, the initial phase of the flare can be
explained by a relativistic jet interacting with the in-
terstellar medium or a torus that creates a shock and
gets frustrated. A relativistic shock was proposed by
Falcke et al. (1999 1 due to synchrotron cooling times of
14-50 days which are much shorter than the duration of
the outburst. The ultra-compact hotspots are pumped up,
powered by the jet and responsible for the increase in flux
density. The post-shock material expands with the max-
imum sound speed of a magnetized relativistic plasma of
c s ps 0.6 c.
Since the source is optically thick at 15 GHz, one ob-
serves the outside of the source, i.e. the post-shock mate-
rial expanding with sound speed. At 43 GHz, the source
is optically thin and one can look inside the source and
see the stationary hotspots.
The rapid expansion at 43 GHz thereafter has marked
the phase where the jet breaks free and starts to propa-
gate relativistically into a lower-density medium. Then the
expansion stops again when the jet hits another cloud.
The fact that spectral and structural evolution dur-
ing the outburst are closely linked demonstrates that we
are dealing with a real physical expansion and not only a
phase velocity. The observations described here produced
10
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
0.05
1998 1998.5
1999 1999.5 2000
Date [years]
2000.5 2001
Fig. 13. Component separation at 43 (upper) and 15
(lower) GHz. The apparent expansion speed is 0.6 c at
15 GHz and 1.25 c at 43 GHz.
a large amount of data that are all consistent with a simple
synchrotron self-absorbed jet model.
For the question of the nature of the radio-
loud/radio-quiet dichotomy this means that radio- weak
and radio-loud quasars can indeed have central en-
gines that are in many respects very similar. Their op-
tical properties are almost indistinguishable and both
types of quasars can produce relativistic jets in their
nuclei. The finding of super luminal motion supports
the hypothesis of Miller, Rawlings, & Saunders (1993)
and Falcke, Patnaik, & Sherwood (19961 that RIQs are
relativistically boosted intrinsically radio-weak AGN.
Recently, a further relativistic jet in a radio-quiet quasar
was found by Blundell, Beasley, & Bicknell (20031.
However, the nature of the medium interacting with
the jet remains unclear. The outbursts could be explained
by a precessing jet that hits a molecular torus roughly
every five years. So far, no direct evidence for molecular
gas in the nucleus of III Zw 2 was found.
Some Seyfert galaxies have shown H2O maser emission
associated with the nuclear jet. In these sources, the maser
emission is the result from an interaction of the jet with
a molecular cloud. One example is the Seyfert II galaxy
Mrk348 fsee Pe ck et al. 2003(1 . In this source, the ejection
of a new VLBI component has lead to a flare of the radio
N
>>
o
c
CD
=5
cr
CD
CD
>
o
c
o
05
c
100
10
0.1
: ' I 1 X' 1
, 1 1 | 1 1 11 1 ' ' ' 1 1 1 ':
X
\X X X
•
X :
MX
" X U X ><
x -K ■ x
■ x
.1 , , , 1
x ■ 1:V "
, , 1 , . . 1 , , , 1 . . . 1 ,
0.01
0.0001 0.001 0.01 0.1 1 10 10c
Projected Linear Size [Kpc]
Fig. 14. Intrinsic turnover frequency vs. linear size for
GPS and CSS sources. The quasars are represented
by crosses, and the galaxies by solid squares. Adapted
from O'Dea & Baum (1997 k The two circles mark the val-
ues for III Zw 2 before and after the expansion at 43 GHz.
source similar to the outburst in III Zw 2. The outburst
started with a peak frequency of ~ 22 GHz which gradu-
ally decreased over 20 months. During this outburst, H2O
maser emission was found ( Falcke ~et al. 2000)1 . Searches
for similar water maser emission in III Zw 2 with the
Effelsberg 100-m telescope yielded no detection (Henkel,
private communication).
In the currently favoured youth model for Compact
Steep Spectrum (CSS) and GHz Peaked Spectrum (GPS)
sources, the linear size of a source is related to the age
of the source. The correlation between the turnover fre-
quency and the projected linear size (e.g., O'Dea & Baum
1997) suggests that the turnover frequency decreases while
the source ages and expands. Therefore the sources with
the highest turnover frequencies represent the youngest
objects. In Fig. 1141 we plot linear size vs. turnover fre-
quency for GPS and CSS sources. We include III Zw 2
before (v ssa ~ 33 GHz; size 0.11 pc) and after (v ssa ~
10 GHz; size « 0.37 pc) the expansion and the two points
lie at the lower end of the scatter of the linear correlation
for GPS/CSS sources. This could be explained by pro-
jection effects. Since III Zw 2 is a Seyfert 1 galaxy with
superluminal motion, the jet is probably close to the line-
of-sight. Hence the true size would be underestimated, and
the points in the plot move to the right. However, the evo-
lution of III Zw 2 during the expansion is almost parallel
to the correlation. This implies that the same physical
processes, i.e. synchrotron self-absorption, are involved in
III Zw 2 and in GPS/CSS sources. In the case of III Zw 2,
the radio source is much older than the current outburst.
Hence, it is possible that some of the GPS/CSS sources
are in fact not young, but only show intermittent activity.
Ill Zw 2 remains an extremely unusual object. Future
simultaneous multi-frequency observations of new out-
A. Brunthaler, H. Falcke, G.C. Bower et al.: Ill Zw 2: Evolution of a radio jet in a Seyfert galaxy
11
bursts would help to confirm the proposed scenario of a
jet-ISM interaction.
Acknowledgements. The National Radio Astronomy
Observatory is a facility of the National Science Foundation
operated under cooperative agreement by Associated
Universities, Inc. The UMRAO is partially supported by
funds from the National Science Foundation and from the
Univ. of Michigan Dept. of Astronomy. The 100 m telescope
at Effelsberg is operated by the Max-Planck-Institut fiir
Radioastronomie in Bonn.
Tiirler M., Courvoisier T. J.-L., Paltani S., 1999, A&A, 349,
45
Taylor G. L., Dunlop J. S., Hughes D. H., Robson E. I., 1996,
MNRAS, 283, 930
Unger S. W., Lawrence A., Wilson A. S., Elvis M., Wright
A. E., 1987, MNRAS, 228, 521
Valtaoja E., Lahteenmaki A., Terasranta H., Lainela M., 1999,
ApJS, 120, 95
Zwicky F., 1967, Adv. Astron. Astrophys., 5, 267
References
Aller H. D., Aller M. F., Latimer G. E., Hodge P. E., 1985,
ApJS, 59, 513
Alonso-Herrero A., Ward M. J., Kotilainen J. K., 1997,
MNRAS, 288, 977
Arp H., 1968, ApJ, 152, 1101
Bahcall J. N., Kirhakos S., Schneider D. P., 1995, ApJ, 450,
486
Blandford R. D., Konigl A., 1979, ApJ, 232, 34
Blundell K. M., Beasley A. J., 1998, MNRAS, 299, 165
Blundell K. M., Beasley A. J., Bicknell G. V., 2003, ApJ, 591,
L103
Boroson T. A., Green R. F., 1992, ApJS, 80, 109
Brunthaler A., Falcke H., Bower G. C, et al., 2000, A&A, 357,
L45
Clements S. D., Smith A. G., Aller H. D., Aller M. F., 1995,
AJ, 110, 529
Falcke H., Biermann P. L., 1995, A&A, 293, 665
Falcke H., Bower G. C, Lobanov A. P., et al., 1999, ApJ, 514,
L17
Falcke H., Henkel C, Peck A. B., et al., 2000, A&A, 358, L17
Falcke H., Patnaik A. R., Sherwood W., 1996, ApJ, 473, L13
Falcke H., Sherwood W., Patnaik A. R., 1996, ApJ, 471, 106
Hutchings J. B., 1983, PASP, 95, 799
Hutchings J. B., Campbell B., 1983, Nat., 303, 584
Kaastra J. S., de Korte P. A. J., 1988, A&A, 198, 16
Kcllermann K. I., Sramek R., Schmidt M., Shaffer D. B., Green
R., 1989, AJ, 98, 1195
Kcllermann K. I., Vermeulen R. C, Zensus J. A., Cohen M. H.,
1998, AJ, 115, 1295
Khachikian E. Y., Weedman D. W., 1974, ApJ, 192, 581
Kirhakos S., Bahcall J. N., Schneider D. P., Kristian J., 1999,
ApJ, 520, 67
Kukula M. J., Dunlop J. S., Hughes D. H., Rawlings S., 1998,
MNRAS, 297, 366
Lloyd C, 1984, MNRAS, 209, 697
Miller P., Rawlings S., Saunders R., 1993, MNRAS, 263, 425
O'Dea C. P., Baum S. A., 1997, AJ, 113, 148
Osterbrock D. E., 1977, ApJ, 215, 733
Peck A. B., Henkel C, Ulvestad J. S., et al., 2003, ApJ, 590,
149
Ryle M. S., Longair M. S., 1967, MNRAS, 136, 123
Salvi N. J., Page M. J., Stevens J. A., et al., 2002, MNRAS,
335, 177
Scarpa R., Urry C. M., Falomo R., Pesce J. E., Treves A., 2000,
ApJ, 532, 740
Schmidt M., Green R. F., 1983, ApJ, 269, 352
Shepherd M. C, Pearson T. J., Taylor G., 1994, BAAS, 26,
987
Surace J. A., Sanders D. B., Evans A. S., 2001, AJ, 122, 2791